TESTING WITH ULTRA VIOLET LIGHT
To buy UV lights, click
here
Ultra violet light (known is the U.S.A. as
'black light') is a very intense blue light, only just visible
to the naked eye. When you switch on a UV light you see a
very dull deep blue glow. In fact, it's so dull that it can
be swamped out completely by bright daylight. The light isn't
really 'dull' at all, it is merely that the human eye has
difficulty detecting it. But what the human eye CAN detect
is the curious effects that occur when UV light is shined
on antiques.
If porcelain has been repaired, the missing
part will have been 'rebuilt' or the crack glued, and then
carefully repainted so that the colour matches. But the colour
will only match perfectly in white light (eg daylight), under
UV light the new paint shows up a totally different shade.
This applies to decorative items, watch dials, dolls heads,
even faint signatures on letters and modern touch-up paint
on old paintings.
How does it work? It works by reflecting
off the surface and showing up the difference in the chemical
composition of the paint. You have probably noticed the same
effect when looking at a car parked under a yellow street
light, new paint on the car can show up quite a different
shade, whereas in daylight it matches perfectly.
There are other examples in every day life.
There is starch in ordinary paper that makes it glow a vivid
blue when lit with ultra violet light, but banknotes (which
are made of a linen-type material and not paper)
merely look 'slightly blue', hence these lights are used as
'forged banknote detectors'. The additives in soap powders
also glow under ultra violet light, they are added for that
very reason: UV light from the sky makes your white washing
glow blue-white, so that the advertisers can say that their
powder 'washes whiter than white'. Most dramatic of all is
Vaseline Glass (Uranium Glass) which is so sensitive to UV
light that it glows brilliantly.
There are also other examples in the world
of antiques and art: oil paintings, art on paper, marble,
jade and ivory all show characteristic patterns under UV light
- scroll down to the bottom to see a chart.
Some questions and answers.
Question: if ultra violet light is 'only
just visible' to the naked eye, how can you see it?
Answer: use it in the dark. This greatly
perturbs some users. How can they use an ultra violet light
in the dark when buying at an antiques fair or in a market
or at auction? Well it doesn't have to be completely dark,
but the darker the better. Use square of dark cloth, or the
underneath of a table in a dim corner or even the inside of
your jacket (you've probably seen dealers lifting their jacket
and, it would seem, examining their armpit with a blue light).
With some of the lights it's enough to shade the item with
your hand. If you have your purchase at home try using the
ultra violet light in pitch darkness, the effect can be quite
dramatic.
Question: if you use it by shining the light
on the surface of an antique, does that mean that it won't
see below the surface?
Answer: exactly so! If the item has been
repaired and then re-glazed, an ultra violet light will be
of no use, there will be no 'new' paint to see, the entire
surface will be new.
To buy any of these online, go to PRODUCTS
Type:
fluorescent bulb - see picture ref. UV
Effectiveness: very effective,
item needs to be shaded e.g. under a square of black cloth
or inside a jacket. This one is often seen mounted under
a shade for testing banknotes, the long bulb covers an area
of about 100mm at a time.
Measurements: 17X5X4mm
Power: four size AA batteries included,
good batteries should last a few hours
Construction / Presentation: plastic,
cardboard box
Type:
'pen' type, small LED bulb mounted just inside the end of
a tiny tube - see picture ref. uv-pen
Effectiveness: requires very dim light (preferably
darkness), you need to work very close to the object and
examine just an area of 5mm at a time, very hard work.
Measurements: 35mm diameter X15mm deep, plus a
keyring fitting.
Power: three LR44 button batteries, good batteries
should last several hours
Construction / Presentation: metal / fitted plastic
box
Price £6.00 -
BUY THIS (bottom item)
Type:
round keyring, large LED bulb sticks out the front - see
picture ref. uv-kr
Effectiveness: very effective,
needs only slightly dim light e.g. shade the item with your
hand, covers an area of about 30mm at a time
Measurements: 5mm diameter X35mm
long plus the keyring fitting
Power: two CR2016 batteries, good
batteries should last a few hundred hours
Construction / Presentation: plastic
with metal rim / plastic bag
Price £15.00 - BUY
THIS (second item down)
